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Nanoplumbing: More than just a pipe dream

By Philip Ball

GERHARD HUMMER was pondering a serious plumbing problem. He was trying to unravel the inner workings of tiny proteins called aquaporins, which are found in the walls of living cells. Each aquaporin is threaded by a narrow pore that helps control the flow of water into the cell. The pore is a complex thing, narrow in parts and wide in others, lined with a variety of chemical groups that mostly repel water. But it is basically a pipe.

And that realisation made Hummer, working at the US National Institutes of Health in Bethesda, Maryland, turn his attention to carbon nanotubes. Consisting of curled-up sheets of carbon and just nanometres wide, they are essentially smooth pipes of water-repelling graphite. Hummer hoped that their simple structure might offer new insights into the way that water travels through aquaporins. It proved a smart move. Nanotubes have not only helped researchers like Hummer understand water flow in proteins, but they are also enabling scientists to devise a host of nanoscale plumbing parts – such as molecular pumps, gates and valves – capable of moving and filtering everything from salty water and hydrocarbon fuels to gases such as carbon dioxide. It seems that these humble tubes could hold the key to cheap desalinated water, better fuel cells and new strategies to tackle global warming.

Hummer’s study of fluid flow in nanotubes kicked off around a decade ago when along with two colleagues he created a detailed computer simulation of the way water moves inside a carbon nanotube just 0.8 nanometres wide. When they dunked the tube into a tiny tank of virtual water, the researchers found that a thin …